Construction machines, such as hydraulic excavators and hydraulic cranes, perform various work by means of a hydraulic drive system. For example, Patent Literature 1 discloses a hydraulic drive system including first and second pumps that supply hydraulic oil to a plurality of actuators and an engine that drives these pumps.
The first and second pumps are variable displacement pumps, and tilting angles of these pumps are adjusted by first and second regulators. A plurality of solenoid proportional valves output secondary pressures to the first and second regulators, and the solenoid proportional valves are controlled by a pump controller.
The engine that drives the first and second pumps includes a fuel injector, and the fuel injector is controlled by an engine controller. The engine controller is connected to a rotation speed selector that receives a selection of a reference rotation speed of the engine (the engine controller is referred to as an “accelerator operation input unit” in Patent Literature 1).
The hydraulic drive system disclosed in Patent Literature 1 is configured such that the engine rotation speed is kept low while the construction machine is performing no work or performing light work, and such that the engine rotation speed increases when an operation device including an operating lever is operated. The operation device is a pilot operation valve that outputs a pilot pressure corresponding to an inclination angle of the operating lever (i.e., outputs a pilot pressure corresponding to the amount of an operation received by the operating lever).
Specifically, first, the pump controller calculates a flow rate control required rotation speed NN and an engine required horsepower PN based on a selected reference rotation speed, a pump discharge pressure, and a pilot pressure outputted from the operation device. The calculated flow rate control required rotation speed NN and the engine required horsepower PN are transmitted from the pump controller to the engine controller. The engine controller calculates a horsepower basis rotation speed NK based on the engine required horsepower PN, and sets a higher rotation speed between the horsepower basis rotation speed NK and the flow rate control required rotation speed NN as a target rotation speed. The engine controller controls the fuel injector, such that the actual rotation speed of the engine is the target rotation speed. For example, when the operation device is not operated, the flow rate control required rotation speed NN is zero. Accordingly, the fuel injector is controlled based on the horsepower basis rotation speed NK.